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206                               CHEMICAL ENGINEERING
known milling machine of this type. Ring-roll mills are high-speed devices and while they have been used for a number of years for all classes of rock reduction, their employment is now limited to dry or wet grinding of comparatively soft materials, such as coal, limestone, mineral earth, etc. The rapid wear of these machines under conditions of hard grinding militates against their use and their high speed makes them a rather difficult machine to control. They are, however, appropriate and economical for use in grinding coal for producing pulverized-coal fuel. They are widely used for this purpose and are said to be satisfactory.
In the case of grinding coal, the removal of the ground material is by an air current which may be regulated to take away the material sufficiently ground and to leave behind the particles which require further grinding. The capacity of a 50- or 54-in. diameter mill for this work will amount to about 1 ton per hour for each muller roll; the machines are usually made either with three, four or five rolls. The power for operation under these conditions varies according to the size of the machine and the excellence of the design, from 25 to 85 hp.
There are variations of the ring-roll type in which the ring is set vertically with rollers pressing against the inner horizontal face of the vertical ring and held there by spring pressure. These variations may have advantages, but such features must be considered with respect to each problem.
Crushing rolls, such as have already been described, are often used for the production of granular rock. A product as fine as 10 or even 20 mesh can be produced by these machines, but for such use they are generally considered uneconomical. Their very small capacity together with their tendency to groove and corrugate, delivering a product of widely varying sizes, makes them rather a high-cost and inefficient machine for such work.
The tube mill or ball tube mill is the machine generally considered most economical for producing either a finely ground product or a finely granular product. The tube mill consists essentially of a tube of varying diameter and length, installed horizontally and rotated about its horizontal axis. Ordinarily the shell is made of plate steel and the heads of cast iron or cast steel. The feed to the mill is through one hollow shaft, upon which the mill is supported and rotates, and the discharge is through the opposite axis. The power is applied through a countershaft equipped with a pinion, to a master gear surrounding the tube, either at the feed or discharge end.
The tube is equipped with a hard-iron or steel lining, which receives the wear and which can be renewed. This lining is either keyed into the mill or bolted in by means of bolts passing through the lining plates and shell. Both heads are also ordinarily lined with segmental lining plates of the same hard material. The grinding is accomplished by placing inside the tube such objects as hard flint, pebbles, cast- or forged-steel balls, cylinders or slugs, depending upon the ideas of the operator. Since the work done depends upon the energy of the grinding medium falling upon the rock to be crushed, it follows that the heavier substance accomplishes the most work and is the most efficient in use.
The tube mill originated in the cement industry, where an extremely fine product is required. Originally flint pebbles were employed as the grinding medium, and the mills were lined with silex or silex brick, a hard siliceous material. Recently, however, it has been found that the efficiency is greatly increased by using a hard-iron or steel liner and steel balls, the output per horsepower being far greater.
The feed is introduced through the hollow trunnion of the mill, in the case of dry grinding assisted by a helix cast as part of the trunnion lining, the screw carrying the